Why are single crystals often preferred over polycrystalline halide ceramics for infrared windows?

Single crystals are preferred over polycrystalline halide ceramics for infrared windows because the grain boundaries and porosity inherent in polycrystalline materials scatter radiation, which reduces optical transmission.1

While halide materials are useful for their ability to transmit lower wavelengths into the infrared region, their polycrystalline forms are less effective due to this scattering effect. Consequently, they are best utilized as single crystals to maintain high transparency. However, because single-crystal halides can be insufficiently strong and may plastically deform under their own weight, they are often hot-forged or fusion-cast to create large-grained structures that resist deformation without significantly compromising infrared transmission.1